The invention relates to an apparatus for the generation of large tensile forces, for instance for pre-stressing screw bolts of pressure vessels of nuclear reactors, of turbine bodies and pump bodies, for the pre-stressing of cables, and for the stressing of various machine parts.
An apparatus for this purpose which is presently used generates the pre-stressing force by means of one or more hydraulic cylinder pistons connected in parallel. These cylinders and pistons are arranged either coaxial of the screw bolt to be stressed, or circumferential of the axis of the screw bolt. The resulting force acts upon an auxiliary nut fixed above them on the end of the screw bolt which extends from a support.
Such prior apparatus has a number of disadvantages. The active surface of the pistons and the working pressure of the pressure medium determine the required tensile force. The lack of space determined by the spacing of the screws, however, limits the diameter of the working pistons when such pistons are arranged coaxial of the screw, and limits the diameter and number of pistons with the arrangement wherein a plurality of pistons are disposed on a circle coaxial with the screw. Thus the resulting force is also limited, and can be increased only by increasing the working hydraulic pressure. In this case, however, problems with the packing of the working pistons and the pressure source increase. In the case of a coaxial arrangement, it is possible to increase the force by increasing the number of pistons connected in parallel; however, with this arrangement problems with packing and number of packing locations also increase. Another drawback of the prior art is that the auxiliary nut, against which the pistons or cylinders rest, is disposed on the prolonged bolt of the screw above the pistons or cylinders, and the prolonged screw bolt passing through the center of the pistons or cylinders. Thus such arrangement occupies a considerable part of the available working surface of the pistons, thereby eliminating the resulting working tensile force.